1. Field of the Invention
The present invention relates to an apparatus and a method of supplying powder coating material, and more specifically, to improvement in response of an actual supply rate to a given supply rate.
2. Description of the Related Art
When powder coating such as coating of a car body is conducted, in order to make uniform the finished coating and to effectively use the coating material, it is necessary to always supply a constant amount of powder coating material to a coating gun. In a conventional apparatus for supplying powder coating material of this type, one end of a carrier tube is inserted in a tank for containing powder coating material. By blowing suction air into a nozzle of an injector provided on the other end of the carrier tube, the powder coating material in the tank is sucked into the carrier tube to be supplied to the coating gun. A measurement device for measuring the flow rate of the powder coating material is attached to the carrier tube. Based on the difference between the actual supply rate of the powder coating material which is measured by the measurement device and a given supply rate which is inputted via a control panel or the like, the pressure of the suction air to be blown into the nozzle of the injector is PID-controlled such that the actual supply rate equals the given supply rate. Here, PID control is a control system which is a combination of proportional operation for smooth control without hunting, integral operation for automatic correction of offset, and differential operation for improved response to disturbance. Such an apparatus for supplying powder coating material enables the coating gun to be always supplied with the given supply rate of powder coating material.
However, since it has been attempted to supply the given supply rate of powder coating material by PID control of the pressure of the suction air to be blown into the nozzle of the injector based on the difference between the actual supply rate and the given supply rate, when the proportional band of the PID control is set to be narrow, response of the actual supply rate to the given supply rate of powder coating material overshoots and oscillates as shown in FIG. 7 by a curve C1. On the other hand, when the proportional band is set to be wide, the response of the actual supply rate to the given supply rate of powder coating material is slow as shown in FIG. 7 by a curve C2. Therefore, as shown in FIG. 8, it takes time to obtain a desired suction air pressure and an actual supply rate in response to change in the given supply rate.
Users of powder coating who have to frequently change the discharge amount of powder coating material desire an apparatus for supplying powder coating material to be developed which can supply powder coating material to a coating gun with good response.
The present invention is made to solve the above problem. An object of the invention is to provide an apparatus and a method of supplying powder coating material with good response of an actual supply rate to a given supply rate.
An apparatus for supplying powder coating material according to the present invention comprises: a tank for containing powder coating material therein; an injector connected to the tank and having a main nozzle; a compressed air source for blowing suction air into the main nozzle of the injector to suck powder coating material from the tank; a powder flow rate measurement device for measuring a flow rate of powder coating material sucked by the injector and supplied from the tank; given supply rate input means for inputting a given supply rate of powder coating material; a suction air pressure regulator for adjusting a pressure of suction air blown into the main nozzle of the injector from the compressed air source; storing means; and a control circuit for storing in said storing means as learned data a value of suction air pressure with respect to a given supply rate in the past, for instantaneously operating, when a new given supply rate is inputted from the given supply rate input means, the suction air pressure regulator such that the suction air pressure equals a value of learned data stored in the storing means with respect to the new given supply rate, for adjusting thereafter the suction air pressure regulator such that a flow rate of powder coating material measured by the powder flow rate measurement device equals the new given supply rate to PID-control the suction air pressure.
A method of supplying powder coating material according to the present invention comprises the steps of: inputting a given supply rate; sucking powder coating material from a tank by blowing suction air into a main nozzle of an injector connected to the tank; measuring a flow rate of powder coating material; adjusting pressure of suction air blown into the injector such that the measured flow rate of powder coating material equals the inputted given supply rate; storing as learned data a value of suction air pressure with respect to a given supply rate in the past; and instantaneously adjusting, when a new given supply rate is inputted, the suction air pressure equals the value of learned data stored with respect to the new given supply rate, and thereafter PID-controlling the suction air pressure such that the measured flow rate of powder coating material equals the new given supply rate.